1. Field of the Invention
This invention relates to an anti-reflection coating for an infrared transmitting material and more particularly, it is concerned with an anti-reflection coating film provided for inhibiting the loss due to reflection in an infrared transmitting material.
2. Description of the Prior Art
Crystalline materials, for example, silver halides such as silver bromide and silver chloride, thallium halides such as KRS-5 and KRS-6 and alkali halides such as cesium iodide and cesium bromide, well known as infrared transmitting materials, have been considered full of promise as CO.sub.2 laser beam transmitting window materials or CO.sub.2 laser beam transmitting fibers. However, these materials each have a large refractive index at a wavelength of 10.6 .mu.m of CO.sub.2 laser beam and accordingly, the incident beam on the surface has a large reflectivity. In Table 1 are shown the refractive indexes and reflectivities of these materials at a wavelength of 10.6 .mu.m of CO.sub.2 laser beam:
TABLE 1 ______________________________________ Materials Index nRefractive (%)Reflectivity ##STR1## ______________________________________ Silver Bromide 2.17 13.6 1.47 Silver chloride 1.98 10.8 1.41 KRS-5* 2.37 16.5 1.54 KRS-6* 2.17 13.6 1.47 Cesium Iodide 1.74 7.3 1.32 ______________________________________ Note: *KRS-5(commercial name): mixed crystal 45.7% TlBr + 54.3% TlI KRS-6(commercial name): mixed crystal of 40% TlBr + 60% TlCl
These materials have a reflectivity of more than 7. Thus, it is important to reduce the reflective loss and to raise the transmission of a window or fiber. As a method of reducing the reflective loss, it is known to form a non-reflective coating film. In the non-reflective coating monolayer, the reflectivity can be made zero by forming on the surface of a substrate a coating film consisting of a material with a refractive index equal to the square root .sqroot.n of the refractive index n of the substrate to give a thickness of d=.lambda./4n wherein .lambda. is the wavelength of a transmitted light. The square root .sqroot.n of the refractive index of the above described materials is in the range of 1.3 to 1.6 as shown in Table 1. As a material transparent in the infrared region and having a refractive index of 1.3 to 1.6, there are potassium bromide, potassium chloride, sodium chloride, sodium fluoride, calcium fluoride and barium fluoride. These materials each have a refractive index and absorption coefficient at a wavelength of 10.6 .mu.m, as shown in Table 2:
TABLE 2 ______________________________________ Absorption Material Refractive Index Coefficient cm.sup.-1 ______________________________________ Potassium Bromide 1.53 4.2 .times. 10.sup.-4 Potassium Chloride 1.45 1.4 .times. 10.sup.-4 Sodium Chloride 1.49 4.5 .times. 10.sup.-4 Sodium Fluoride 1.23 5 .times. 10.sup.-2 Calcium Fluoride 1.31 3.5 Barium Fluoride 1.42 0.19 ______________________________________
The reflectivity can be decreased to less than about 3% by forming a film of these materials with a thickness of .lambda./4n on the surface of silver bromide, silver chloride, KRS-5, KRS-6 or cesium iodide. However, calcium fluoride and barium fluoride each having a relatively large absorption coefficient as shown in Table 2 are not suitable for use in transmitting high output CO.sub.2 laser beam, since their coatings tend to be damaged due to absorption. On the other hand, potassium bromide, potassium chloride, sodium chloride and sodium fluoride are all deliquescent although their absorption coefficients are sufficiently small, so the coatings thereof tend to take moisture in the air and to be readily decomposed and cannot be used as an anti-reflection coating in stable manner for a long time.